Chlorine dioxide is utilized in a variety of bleaching operations, particularly in the bleaching of cellulosic fibrous material, such as, wood pulp produced by the Kraft and sulfite processes.
One process for the production of chlorine dioxide is described in Canadian Patent No. 913,328 in which sodium chlorate is reacted with hydrochloric acid which provides the acid medium and the reducing agent. The process, when sodium chlorate is reduced, is represented by the equation: EQU NaClO.sub.3 + 2HCl .fwdarw. ClO.sub.2 + 1/2Cl.sub.2 + NaCl + H.sub.2 O.
this chlorine dioxide-producing reaction is carried out at low acidities of the reaction medium, in the range of about 0.05 to about 1 normal, preferably about 0.05 to about 0.5 normally, particularly about 0.1 to about 0.2 normally.
In the processes described in this Canadian patent, the evaporation is carried out at the boiling point of the reaction medium under a reduced subatmospheric pressure to deposit product sodium chloride in the reactor after the byproduct has built up to saturation of the reaction medium and the liquid level in the vessel generally is maintained at a substantially constant level by balancing the water added with that evaporated. The evaporated water acts as diluent gas for the generated chlorine dioxide and chlorine and removes the gaseous products from the reaction vessel.
The present invention is directed to a particular manner of carrying out this chlorine dioxide-producing process.
The reaction vessel containing the reaction medium may be of any convenient design and usually includes a cylindrical body with a conical base. A conduit for removal from the vessel of solid product in a slurry with some reaction medium communicates with the conical base. Solid material is removed from the slurry and make-up chemicals are added to the remaining material before recycle to the reaction vessel. The recycling liquid is heated in a heat exchanger, typically to substantially the boiling temperature of the solution under the prevailing conditions of the reaction vessel, prior to discharge to the reaction vessel.
In conventional evaporator practice in which a liquid is evaporated in a vessel, a slurry is removed, solid is recovered and liquid is recycled and reboiled during recycle, the liquid is prevented from boiling in the recycle line by operating the vessel with the level of liquid above the entry point of the recycled liquid a height greater in hydrostatic head than the increase in saturated vapour pressure caused by the temperature rise across the recycle loop heater.
However, in the chlorine dioxide-producing process used in the present invention, since there is reaction to form chlorine dioxide upon addition of hydrochloric acid to the recycling chemicals in the recycle loop it is not possible to have the level of liquid in the reaction vessel above the discharge point of the recycled liquid. The recycled material therefore, is discharged above the level of the liquid in the reaction vessel.
The velocity of the liquid in the recycle loop and discharging to the reaction vessel above the liquid level therein must be maintained above a predetermined minimum value to prevent boiling of the liquid in the heat exchanger and to prevent vibration due to flashing flow acceleration. This minimum velocity is high and as a result the discharging material may impinge on the opposite wall of the vessel. Due to the solids content of this material, the vessel wall is subject to wear by the impinging material.